The present invention relates to an AC generator control device and more particularly to an AC generator control device of the type in which an electric current flowing through a field coil is controlled by turning a regulator on and off.
In conventional AC generators, electric current flows through a field coil and is controlled by turning a regulator on and off. The duty ratio of a pulse signal developed at the junction of the field coil and the regulator and taken out through a field current state output terminal (F terminal) is used for a predetermined control. An example of the operation of such a generator in an automotive context is described below.
When a headlight is lit during idling of the engine, the field current increases to increase the power output of the AC generator in accordance with the increase of the load current. That is, the duty ratio of the pulse signal becomes larger. As the generated output increases, the load imposed on the engine also increases. If no countermeasure is taken, it is possible that the idling speed will diminish, or that stalling of the engine will occur.
To avoid such occurrences, the duty ratio of the pulse signal developed at the junction of the field coil and the regulator is detected by a voltage detection circuit connected to the F terminal and the throttle valve opening is controlled according to this detected value to keep the idling speed constant.
In such a generator, since a line is extended from the F terminal to the voltage detection circuit, a short-circuit can occur between the line and ground. It is apparent that if such a short-circuit were to occur, there would be an uncontrollable flow of current in the field coil and the voltage output of the AC generator from the output terminal (B terminal) would increase to an abnormal extent. A battery charged by such abnormal output voltage might be damaged as a result, as would an electrical device operated by such output voltage.
When the engine is turned off under such abnormal conditions, the field current will continue to flow from the battery through the B terminal of the generator, the field coil, the F terminal and the short-circuited portion. As a result, the battery capacity would be reduced, or the battery would become overly discharged.